The invention relates to steam generators and in particular to an apparatus for limiting economizer heat exchange during start-up and low load operation.
On steam generators used for rapid start-up, the heavy firing leads to considerable heat transfer to the water in the economizer. When the unit is at subcritical pressures during this time, the overheating results in the formation of steam in the economizer. Where the economizer flow is carried to a steam drum, this results in water hammer while where the flow is introduced directly into waterwall circuits, as in a once-through unit, this produces a problem of distributing a two-phase mixture of water and steam.
One solution has been to increase the feedwater flow to the extent that steam is not formed. This requires additional blowdown capacity in the unit to discharge the excess feedwater flow. Where a motor driven start-up feed pump is used, this may exceed the capacity of the pump. It also requires additional deaerating steam to deaerate the excess flow. Furthermore, any tolerance which is applied in the saturation temperature increases the heat absorption duty of the furnace since more water must be heated up to saturation; and this amount includes not only the water that is desired to be evaporated but the excess flow as well.
A gas bypass duct around the economizer would decrease the economizer heat absorption and thereby avoid steaming. This duct must be designed to handle hot, dust-laden gas and must operate in that atmosphere. It is difficult to maintain properly-operating and tightly-sealed dampers in such an environment.
Furthermore, the only pressure difference available for passing the gas through the bypass duct would be the draft loss of the remaining gases through the economizer (and through the gas side of the air heater if the duct also bypasses the air heater) thereby requiring a large duct.
Another possible approach on steam generators having recirculation through the furnace walls is to recirculate some of the saturated water from the steam drum through the economizer. This raises the economizer water temperature and lowers the log mean temperature difference between the gas and the water. This tends to reduce the heat absorption but has an upper limit on the ability to avoid steaming, this limit, of course, being a function of the pressure and the concomitant saturation temperature.